1. Field of the Invention
This invention relates to a recording device for monitoring the integrated time-temperature or radiation-dosage history of an article.
2. Background of the Invention
Time-temperature history indicators for measuring the integrated thermal exposure of articles such as perishable foods, pharmaceuticals, photographic film, blood and the like are well known in the art.
A recording device comprised of a series of temperature-sensitive, color-responsive liquid crystals is disclosed in U.S. Pat. No. 3,661,142 (Flam, 1972) wherein a series of varying liquid crystal compositions undergoes color changes at different temperatures. However, this device does not permanently record time-temperature history since the liquid crystals are thermochromic and undergo reversible color changes.
Other recording devices based upon color changes associated with varying composition are disclosed in U.S. Pat. No. 3,871,232 (Pickett et al., 1973) in which solids having different melting points solubilize a dye upon melting, thus indicating the maximum temperature exceeded; and U.S. Pat. No. 3,700,603 (Rembaum, 1972) which employs complexes of electron-donating and electron-accepting organic compounds to form colored complexes in the liquid state to indicate that a particular temperature has been exceeded. However, both these devices record the maximum temperature achieved rather than the integrated time-temperature history.
A time-temperature integrating indicator is disclosed in U.S. Pat. No. 3,768,976 (Hu et al., 1973) in which a redox dye is colored in its reduced state but turns clear in the oxidized state due to the diffusion of oxygen into the device over a predetermined time interval. The disappearance of the color reveals a message such as "Don't eat this food." The indicator can be set for various predetermined times at various temperatures and functions therefor as an integral time-temperature history indicator. However, the device suffers from the disadvantage of depending on diffusion rates of oxygen to deactivate the indicator wherein the diffusion rates are subject to many variations. Most important, this device indicates whether or not a particular integral time-temperature exposure has been exceeded, but does not clearly indicate the precise value of this exposure.
A time-temperature indicating device is disclosed in U.S. Pat. No. 3,344,670 (Olson et al., 1967) comprising paper impregnated with deposited silver nitrate, which upon exposure to temperature, changes color wherein the time-temperature history can be determined by reference to a calibrated color chart. However, this indicator suffers from the disadvantage that the color transitions are gradual rather than sharp and the active ingredient is expensive.
Polyacetylenes having at least two conjugated C.dbd.C groups have been disclosed in U.S. Pat. No. 3,822,134 (Rasch et al., 1974) for use as vacuum deposited radiation sensitive elements which undergo a color change upon exposure to high temperatures or radiation.
Indicators have been disclosed in U.S. Pat. No. 3,999,946 (Patel et al., 1976), which describes utilizing acetylenic compounds containing at least two conjugated C.tbd.C groups for measuring time-temperature histories of a large number of perishable products. Also disclosed is the use of gamma radiation at liquid nitrogen temperature, -196.degree. C., in order to dramatically alter the reactivity rate of several diacetylenes.
The reference, Di Macromolekulare Chemie 145 (1971) pp. 85-94, describes the thermal polymerization characteristics of bis(p-toluenesulfonate) of 2,4-hexadiyn-1,6-diol. However, the reference does not discuss the utility of prereacting this particular diacetylene to a known extent to form an end-point color in a predetermined time when thermally annealed.
The reference, U.S. Pat. No. 3,501,302 (1970) describes irradiating photosensitive crystalline polyacetylene compounds with ultraviolet radiation and measuring the time required for a given ultraviolet radiation dosage to develop the compound to a known color standard. However, no mention is made of the possibility of using this technique for prereacting the diacetylene compounds for forming an indicator tab in which the time required to produce the final end-point color has been decreased to a known extent.
However, a wide variety of perishables exist which require different indicators because each possesses a different time-temperature history related shelf life which is critical and must be accurately monitored in order to insure quality.
What is needed and what the prior art does not provide are devices suitable for measuring the integrated time-temperature or radiation-dosage histories of a wide variety of products by recording indicators, which can be manufactured from a relatively small number of inexpensive materials.